From The Deluxe Food Lover's Companion
Yeast is a living, microscopic, single-cell organism that, as it grows, converts its food (through a process known as fermentation) into alcohol and carbon dioxide.

From Dictionary of Food: International Food and Cooking Terms from A to Z
Dough formed into a ball, cross cut on the top, corners pulled out and rolled out to a square with a thick centre and thin corners.